This invention relates to a level measuring device operating with microwaves.
To measure level in vessels, for instance in a liquid tank or a bulk container, particularly quasi-continuously, measuring devices operating with microwaves are frequently employed. In the measurement of level with such a level measuring device, which is based on the radar principle, electromagnetic waves, particularly in a center-frequency range of approximately 0.5 to 30 GHz, are transmitted as space waves via an antenna or as guided waves via a surface waveguide against a substance whose level is to be measured. Because of electrical impedance discontinuities within the volume comprising substance, the waves are partially reflected, particularly from the surface of the substance, and are returned via the surface waveguide or the antenna to the level measuring device as an echo signal.
In such level measuring devices, the determination of level is frequently based on the pulse radar method, in which short microwave pulses, so-called bursts, are transmitted with a pulse repetition rate in the range of a few megahertz against the substance, at least partially reflected there, and returned as an echo signal to the level measuring device in the manner described above. A transit time of the microwave pulses, measured from the time the signal is transmitted to the time the echo signal is received, serves as a measure of the level to be determined.
U.S. Pat. No. 5,614,911, EP-A 955 527, and DE-A 44 07 369, for example, each show a level measuring device operating with microwaves which uses the pulse radar method and comprises:
a microwave transceiver unit for generating a level-dependent analog intermediate-frequency signal by means of a pulsed transmit signal and a receive signal;
a transducer element
which in operation couples waves into the vessel under control of the transmit signal, and
which converts echo waves reflected from contents of the vessel into the receive signal; and
an envelope evaluation unit with an amplitude demodulator for the intermediate-frequency signal for generating an analog envelope signal.
As described in DE-A 44 07 369, the envelope signal may serve to control a threshold-triggered counter stage in such a way that an average value of two counts of the counter stage represents the level-dependent transit time. As described in EP-A 955 527, for example, the analog enevelope signal may also be digitized and, after being periodically modulated with a square-wave window, stored temporarily in sections. From an envelope sampling sequence thus generated, both the time of transmission of the signal and the time of reception of the echo signal can be determined and the transit time can be calculated therefrom using a suitable evaluating technique, particularly a technique implemented in a microcomputer.
It turned out that in order to determine level with a high degree of accuracy, in particular accurately to a millimeter, in addition to the amplitude information of the intermediate-frequency signal, which is mapped onto the envelope signal, information about the phase of the receive signal relative to that of the transmit signal may be necessary, as described in EP-A 1 069 438 or in DE-A 44 07 369, for example. To derive such additional phase information, the level measuring device disclosed in DE-A 44 07 369 further comprises a phase evaluation unit with an analog quadrature demodulator for the intermediate-frequency signal to generate an analog first quadrature signal, representing the real part of the intermediate-frequency signal, and an analog second quadrature signal, representing the imaginary part of the intermediate-frequency signal.
A major disadvantage of that level measuring device is the discrete design of both the envelope evaluation unit and the phase evaluation unit. Because of this design, particularly because of the great share of analog devices, an increase in the pulse repetition rate of the transmit signal and/or a reduction of the measurement and evaluation cycle time, for example in order to increase measurement accuracy and/or the speed of evaluation, is only possible on a very small scale. Furthermore, to ensure sufficient accuracy of the transit time determined, each of the components used must both belong to a type class with small variances in component parameters and with high long-term stability, and be wired with high accuracy and, consequently, at high cost.
It is therefore an object of the invention to provide a level measuring device, particularly a device measuring accurately to a centimeter, which uses both amplitude information and phase information for the level measurement and permits a significant increase in the evaluating speed during the level measurement. Furthermore, the level measuring device is to be suited for large-scale integration.
To attain the object, a first variant of the invention provides a level measuring device operating with microwaves, particularly with microwave bursts, for producing a level value representative of a level in a vessel, said level measuring device comprising:
a transceiver unit for generating a level-dependent intermediate-frequency signal by means of a transmit signal and a receive signal;
a transducer element
which in operation couples waves, particularly pulsed waves, into the vessel under control of the transmit signal and
which converts echo waves reflected from contents of the vessel into the receive signal; and
a control unit with a volatile data memory for storing, at least temporarily, a sampling sequence representing the intermediate-frequency signal.
A second variant of the invention provides a level measuring device operating with microwaves, particularly with microwave bursts, for producing a level value representative of a level in a vessel, said level measuring device comprising:
a transceiver unit for generating a level-dependent intermediate-frequency signal by means of a transmit signal and a receive signal;
a transducer element
which in operation couples waves, particularly pulsed waves, into the vessel under control of the transmit signal and
which converts echo waves reflected from contents of the vessel into the receive signal; and
a control unit with a volatile data memory for storing, at least temporarily, a digital phase sequence
which represents a normalization of the intermediate-frequency signal to an amplitude variation of the intermediate-frequency signal and
which corresponds to a temporal phase variation of the intermediate frequency signal.
In a first preferred embodiment of the two variants of the invention, the level measuring device comprises a logarithmic amplifier for the intermediate-frequency signal.
In a second preferred embodiment of the invention, the level measuring device determines the level value by means of amplitude information derived from the sampling sequence.
In a third preferred embodiment of the invention, the level measuring device determines the level value by means of phase information derived from the sampling sequence.
In a fourth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a first signal sequence, which represents a numerically performed multiplication of the sampling sequence by a digital sine-wave signal sequence, and/or a second signal sequence, which represents a numerically performed multiplication of the sampling sequence by a digital cosine-wave signal sequence.
In a fifth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a first quadrature-signal sequence, which represents a numerically performed downconversion of at least a portion of the first signal sequence, and/or a second quadrature signal sequence, which represents a numerically performed downconversion of at least a portion of the second signal sequence.
In a sixth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a first average-value sequence, which serves in particular to generate the first quadrature-signal sequence and represents a variation of a time average of at least a portion of the first signal sequence, and/or a second average-value sequence, which serves in particular to generate the second quadrature-signal sequence and represents a variation of a time average of at least a portion of the second signal sequence.
In a seventh preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a data record which corresponds to a phase of a data record of the sampling sequence and represents a numerical division of a data record of the first quadrature-signal sequence by an essentially equal-locus data record of the second quadrature-signal sequence.
In an eighth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a first digital phase sequence which corresponds to a temporal phase variation of at least a portion of the intermediate-frequency signal.
In a ninth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a digital envelope which represents a temporal amplitude variation of the intermediate-frequency signal.
In a tenth preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a data record which corresponds to a phase of a data record of the sampling sequence and represents a numerical division of said data record by an essentially equal-locus data record of the envelope.
In an eleventh preferred embodiment of the first variant of the invention, the volatile data memory holds, at least temporarily, a second digital phase sequence which corresponds to a temporal phase variation of at least a portion of the intermediate-frequency signal.
A fundamental idea of the invention is to generate digitally stored raw data, i.e., data with a maximum possible content of information about the measured volume, particularly about the level of the contents of the vessel, and with a minimum possible content of analog preprocessing, on which subsequent evaluation procedures, particularly those for determining level, are based. The invention is also predicated on the surprising recognition that despite increased memory requirements, a significant improvement in evaluating speed can be achieved at high accuracy.
One advantage of the invention is that both the conversion of the intermediate-frequency signal and the processing of the sampling sequence can be adapted in a very simple manner to changing boundary conditions, such as a change in pulse repetition rate, a drift of the center frequency of the transmit signal, or a change in evaluation cycle time.
Another advantage of the invention is that the amount of circuitry required to ensure a sufficient signal-to-noise ratio of the intermediate-frequency signal to be digitized is greatly reduced.